Researchers have demonstrated a device that can swap quantum excitation states …

All sorts of counterintuitive behavior happens with regularity in the quantum realm, but very little of that bleeds over into the world of classical mechanics that the human senses occupy. We can register the effects of the quantum behavior of electrons and atoms, but the actual objects that undergo tunneling and entanglement are invisible to the naked eye. In the last couple of years, however, researchers have started working with mechanical oscillators that can display quantum behavior in some circumstances. A paper that will be released by Nature now provides pretty unambiguous evidence for quantum interactions between a standard qubit and a piezoelectric device that's roughly 50µm long—large enough to be seen with the naked eye.

This isn't the first paper to describe quantum behavior in a mechanical device, but it seems to be one of the cleanest. For the most part, the work has focused on microscopic levers, where the vibrational modes can be characterized in terms of a quantum mechanical unit called a phonon. The number of modes accessible increases rapidly as temperature goes up, which is why vibrations never appear to be quantum mechanical in our day-to-day experience.

To study a system like this at the quantum level, you need to ensure that very few of these vibrational modes are accessible—ideally, you want to be able to bring the system to its ground state, and excite it with the lowest-energy phonon possible. Unfortunately, the ground state temperature is related to the vibrational frequency. To reach it, you either need to reach temperatures below those possible with current refrigeration methods, or have something that can vibrate incredibly quickly.

Instead of opting for the reinvention of refrigeration technology, the authors of the new paper, based at the University of Santa Barbara, chose the alternative: they designed a mechanical device that oscillates at a frequency of roughly 6GHz. The device involves a pair of aluminum electrodes that sandwich a layer of aluminum nitride, which is a strong piezoelectric material (it can convert physical strain to current and vice versa). The oscillator can be pushed out of its ground state either by adding current or by absorbing microwaves of the appropriate frequency. All together, the device is over 50µm, which the authors state is visible to the naked eye.

When chilled down to its ground state at 25 milliKelvin, the piezoelectric device can start oscillating when it receives energy via either microwaves or current. The challenge then was determining that these oscillations reflected the presence of phonons.

The layout of the device, with the Josephson flux qubit on top, the piezoelectric oscillator at bottom, and a capacitor linking the two (middle).Image Courtesy of Andrew Cleland

To perform that test, the authors linked the oscillator to a standard quantum electrical circuit called a Josephson phase qubit, which contains (surprise!) a Josephson junction—two superconducting layers that flank a thin layer of non-superconducting material. The Josephson junction, when linked with a capacitor and inductor, can form a qubit, and store quantum information via a ground and excited state. The frequency of transitions between these states is adjustable, and covers the range between five and 10GHz, which nicely overlaps the resonant frequency of the piezoelectric oscillator. This frequency is adjustable on the fly, meaning the authors could switch the interactions between the two devices on and off at will.

The experimental device linked the two systems through a capacitor. When the two were linked, the authors could demonstrate that the behavior of the Josephson qubit was strongly sensitive to the microwave resonant frequency of the oscillator—in fact, as shown at top, their measurements were an almost perfect match for the predictions. In short, the classical mechanical resonance was creating a feature that could be detected with a quantum device.

The authors were able to demonstrate that the average time before a quantum of energy was lost was in the neighborhood of 17 nanoseconds. Fortunately, the typical energy exchange between the two systems only took about four nanoseconds, so they were able to excite the Josephson qubit and watch the excited state hop to the oscillator (where it appeared as a phonon) and back several times. Again, the observed behavior was an excellent match to the calculated predictions.

For those who got lost in the details, the short version: the device can convert a quantum state to mechanical oscillations and back. And (in the grand scheme of things), it's big.

The short, 17ns period before the energy is lost from the system means that there's not enough time for a careful study of the entanglement between the two systems. The qubits are normally stable for 500ns, though, which leads the authors to suggest that the energy is lost through the aluminum nitride layer. That gives them an obvious next step, namely getting the system to survive long enough for some detailed studies.

An accompanying News & Views article by Markus Aspelmeyer also suggests that it might be possible to use the principles behind this device to either bring the frequencies needed down, or to increase the physical motion involved in the mechanical resonator. If we can manage both, Aspelmeyer suggests, then it might be possible to determine whether a quantum state could actually involve having a physical device appear in two places at once.

Aspelmeyer's article is also worth reading because he describes how the physics community first became aware of the potential for this work: Andrew Cleland, the last author of the paper, dropped it into the last slide of a talk he gave last July. There was apparently dead silence as the implications sunk in, and then a round of roaring applause from the audience.